The present invention relates to thermal sensing, and more specifically to methods and apparatus for a programmable thermal sensor in an integrated circuit.
Advances in silicon process technology has lead to the development of increasingly larger die sizes for integrated circuits. The large dies sizes permit integration of millions of transistors on a single die. As die sizes for integrated circuits become larger, the integrated circuits consume more power. In addition, advances in microprocessor computing require execution of a large number of instructions per second. To execute more instructions per second, the microprocessor circuits operate at an increased clock frequency. Therefore, a microprocessor containing over one million transistors may consume over 30 watts of power. With large amounts of power being dissipated, cooling becomes a problem.
Typically, integrated circuits and printed circuit boards are cooled by either active or passive cooling devices. A passive cooling device, such as a heat sink mounted onto an integrated circuit, has a limited capacity to dissipate heat. An active cooling device, such as a fan, is used to dissipate larger amounts of heat. Although a fan cooling system dissipates heat, there are several disadvantages associated with such a system. Traditionally, fans cool integrated circuits by air convection circulated by a fan. However, when a fan is used in conjunction with a high density multi-chip computer system, a large volume of air is required for cooling thereby necessitating powerful blowers and large ducts. The powerful blowers and large ducts implemented in the computer occupy precious space and are too noisy. The removal of a cover or other casing may result in a disturbance of air flow causing the fan cooling system to fail. In addition, the fan cooling system is made up of mechanical parts that have a mean time between failure (MTBF) specification less than a typical integrated circuit. Furthermore, fan cooling systems introduce noise and vibration into the system.
In addition to cooling systems, thermal sensors are implemented to track the temperature of an integrated circuit or electronic system. Typically, thermal sensors consist of a thermocouple which is directly attached to a heat sink. In more sophisticated thermal sensing systems, a diode and external analog circuitry are used. In operation, the voltage/current characteristics of the diode change depending upon the temperature of the integrated circuit, and the external analog circuitry measures the voltage or current characteristics of the diode. The additional analog circuitry is complex and difficult to implement. In addition, employing the analog circuitry results in a thermal time delay degrading the accuracy of such a configuration. Moreover, external analog circuitry for sensing the voltage of the diode consumes a larger area than the integrated circuit being sensed. Therefore, it is desirable to provide a thermal sensor which is incorporated into the integrated circuit. In addition, it is desirable to provide a thermal sensor that can provide feedback for an active cooling system. Furthermore, it is desirable to control the temperature of an integrated circuit without the use of a fan. The present invention provides an integrated thermal sensor that detects a threshold temperature so that active cooling of the integrated circuit is accomplished through system control.
A programmable thermal sensor is implemented in an integrated circuit. The programmable thermal sensor monitors the temperature of the integrated circuit, and generates an output to indicate that the temperature of the integrated circuit has attained a predetermined threshold temperature. The programmable thermal sensor contains a voltage reference, a programmable Vbe, a current source, and a sense amplifier or comparator. The current source generates a constant current to power the voltage reference and the programmable Vbe. With a constant current source, the voltage reference generates a constant voltage over varying temperatures and power supply voltages. In a preferred embodiment, the voltage reference is generated with a silicon band gap reference circuit. The constant voltage from the voltage reference is one input to the sense amplifier. The programmable Vbe contains a sensing portion and a multiplier portion. In general, the programmable Vbe generates a voltage dependent upon the temperature of the integrated circuit and the value of programmable inputs. The programmable inputs are supplied to the multiplier portion to generate a multiplier value for use in the multiplier portion. The voltage reference is compared with the voltage generated by the programmable Vbe in the sense amplifier. The sense amplifier generates a greater than, less than, signal.
The programmable thermal sensor of the present invention is implemented in a microprocessor. In addition to the programmable thermal sensor, the microprocessor contains a processor unit, an internal register, microprogram and clock circuitry. The processor unit incorporates the functionality of any microprocessor circuit. The clock circuitry generates a system clock for operation of the microprocessor. In general, the microprogram writes programmable input values to the internal register. The programmable input values correspond to threshold temperatures. The programmable thermal sensor reads the programmable input values, and generates an interrupt when the temperature of the microprocessor reaches the threshold temperature. In a first embodiment, the interrupt is input to the microprogram and the processor unit. In response to an interrupt, the processor unit may take steps to cool the temperature of the microprocessor, and the microprogram programs a new threshold temperature. For example, the processor may turn on a fan or reduce the clock frequency. The new threshold temperature is slightly higher than the current threshold temperature so that the processor unit may further monitor the temperature of the microprocessor.
In a second embodiment of the present invention, the interrupt generated by the programmable thermal sensor is input to external sensor logic. The external sensor logic automatically controls the frequency of the microprocessor. If the temperature of the microprocessor raises, then the clock frequency is decreased. Conversely, if the temperature of the microprocessor drops, then the system clock frequency is increased. In addition to a programmable thermal sensor, the microprocessor contains a fail safe thermal sensor. The fail safe thermal sensor generates an interrupt when detecting that the microprocessor reaches predetermined threshold temperatures and subsequently halts operation of the system clock. The predetermined threshold temperature is selected below a temperature that causes physical damage to the device. The microprocessor of the present invention is implemented in a computer system. Upon generation of an interrupt in the programmable thermal sensor, a message containing thermal sensing information is generated and displayed to a user of the computer system.